The method for producing olive oil consists in crushing (milling) whole olives until a fine paste or pulp is obtained. During this step, the milled olive material can be constantly washed with water. The paste is then mechanically pressed (pressing) in order to extract therefrom its liquid content. The liquid obtained is then decanted in order to separate the two phases of which it is composed: the oil and the aqueous phase. The waters from washing the milled material and the aqueous phase that are recovered at the end of the pressing are called wastewaters or alternatively vegetable waters. These waters represent approximately 60% of the weight of the olive.
These wastewaters are very rich in antioxidant phenolic compounds; an antioxidant phenolic compound that is particularly desired is hydroxytyrosol; it is present at a content of 0.1 to 2 g/l in the vegetable waters.
Hydroxytyrosol is a simple phenolic compound which has a side chain of alcohol type at position 1 of the aromatic ring, and also two hydroxyl substitutions at positions 3 and 4 of said ring. This compound belongs to the family of ortho-diphenols, which are compounds generally known to have advantageous antioxidant properties.
Its consumption appears to contribute to the beneficial effect of the Cretan diet on human health. Numerous research studies carried out on hydroxytyrosol have shown the advantage of this compound as an antioxidant. The antioxidant activity of hydroxytyrosol confers thereon fatty-substance-preserving agent properties; it is also known for its antibacterial, antiviral and antifungal properties.
This compound is therefore considered to be a very good food supplement or additive and to be an active agent that is of use for reinforcing cell protection with respect to oxidative stress and to the pathological conditions that it generates.
Owing to the advantage, in particular for the food and cosmetics industries, that hydroxytyrosol, and more generally olive polyphenols, represent, various attempts at recovering the by-products rich in phenolic compounds, such as wastewaters (Visioli et al. “Waste waters from olive oil production are rich in natural antioxidants” Experientia, 1995; 51: 32-34), have been carried out.
U.S. Pat. No. 6,361,803 thus describes a method for extracting antioxidant compositions from olive-derived products: pulp, oil or alternatively wastewaters from olives at various stages of maturity. When the starting product consists of wastewaters, the method consists in treating them on an adsorbent polymeric resin which traps the antioxidant compounds, and then in washing the resin with an organic polar solvent in order to recover the antioxidant compounds.
However, the implementation of this method appears to be less suitable for the wastewaters than for the other olive-derived products. Specifically, comparison of the phenolic-compound composition of extracts obtained from various starting materials shows that the extract obtained from wastewaters has a low phenolic-compound content: 16.8%; in addition, the antioxidant activity of this extract is the lowest of all the extracts obtained.
European patent application EP 1 623 960 describes a method for preparing tyrosol and/or hydroxytyrosol from olive wastewaters (vegetable waters) which comprises steps of:                physical separation involving several membrane treatments (microfiltration, ultrafiltration, nanofiltration and reverse osmosis) of the vegetable waters, preferably at a neutral or alkaline pH;        chromatographic separation of the tyrosol, of the hydroxytyrosol and of other phenolic compounds from the concentrate obtained after the final membrane treatment;        catalytic conversion of the tyrosol to hydroxytyrosol in the presence of a mixture of methylrhenium trioxide (MTO) and of hydrogen peroxide;        recovery of the aqueous phase comprising the antioxidant phenolic compounds.        
This method has the drawback of implementing a chemical hydroxylation of the tyrosol to give hydroxytyrosol; however, in view of the subsequent consumption of the hydroxytyrosol obtained, it is necessary to eliminate the catalysts, in particular the methylrhenium trioxide, which is a strong irritant. In addition, this method comprises a very large number of steps: example 1 of this document implements a method which comprises no less than 13 membrane treatments. It is therefore very long to implement and expensive in terms of technical and human means, and its proportioning to the industrial scale is not economically envisionable.
The same is true for the method described in international application WO 2005/123603, which is intended for the fractionation of olive wastewaters with the aim of recovering them. This method comprises a step of adjustment of the pH; of enzymatic hydrolysis of the cellulose, hemicellulose and pectin contained in the wastewaters; of centrifugation; of tangential microfiltration; of tangential ultrafiltration; of diafiltration; of tangential nanofiltration and of reverse osmosis.
International application WO 2007/013032 describes a method for obtaining hydroxytyrosol-rich concentrates in particular from wastewaters; this method combines at least two steps:                an extraction of hydroxytyrosol and of other bioactive compounds by means of a supercritical fluid and/or by nanofiltration; and        a reverse osmosis.        
The extraction by means of a supercritical fluid requires the use of very high pressures (above 74 bar when CO2 is used as supercritical fluid), and it is therefore a method that is complex and also expensive to implement. In addition, reverse osmosis is a process which results in the concentrating of an extract through removal of water; in particular, reverse osmosis does not allow a selective enrichment with low-molecular-weight phenolic compounds.
U.S. Pat. No. 6,416,808 describes a method for obtaining a hydroxytyrosol-rich composition from olive wastewaters, comprising the following steps: (i) production of a wastewater from depitted olives; (ii) addition of a sufficient amount of acid, preferably of citric acid, to reach a pH of between 1 and 5; (iii) incubation of the acidified wastewaters for at least two months in order for the oleuropein to be hydrolyzed to hydroxytyrosol. The method can also comprise a step of extraction of the hydroxytyrosol with an organic solvent, a high performance liquid chromatography (HPLC) or alternatively an extraction with supercritical fluid.
Although it produces hydroxytyrosol-enriched compositions, this method, when it implements an extraction by HPLC or with supercritical fluid, is too expensive to be used on the industrial scale. In addition, the liquid-liquid extraction uses toxic organic solvents which are not compatible with use of the extract obtained in the cosmetics or food industries.
Thus, the methods described in the prior art do not give complete satisfaction and there remains the need to develop a method for extracting low-molecular-weight phenolic compounds from olive wastewaters which is simple and suitable for industrial implementation.